Nebula
A nebula (plural nebulae, not nebulas), is a cosmic cloud of gas and dust. Structure There are accepted 4 main types of nebulae: * Dark nebula which emits no visible light, but blocks light from stars behind. * Luminous nebula (also divided into reflectivity nebula, glowing nebula and H II nebula) which can be seen to the naked eye. * Planetary nebula which is produced by dying Red giants. * Supernova nebula which is produced by supernovas. The first two may last for millions or billions of years. The last two last for thousands of years, until they are absorbed in interstellar space. It is important to say that matter in a nebula is very diffuse. A cloud with the same volume as Earth will only consist of a few kg of matter. A denser cloud will spread across space or will collapse under its own weight. Nebulae are visible due to their huge sizes (some of them may cover millions of light years), while some of them are not visible with naked eye, even if you stay very close to them. So, the image in science fiction where a ship hides within the clouds of a nebula is not quite plausible. Even if matter is so rarefied, a nebula is not a quiet place. Particles might have speeds of thousands of km per second, gas can be very hot (up to 30000 degrees K) and strong magnetic fields have been confirmed. Large nebulae Large nebulae stretch from a few to millions of light years. In past, all luminous nebulae were considered hot nebulae, while all dark ones were listed as cold nebulae. However, this is not the truth. In many cases, inside a dark nebula stars can be forming and you can witness high temperatures, but the outer clouds are cold and obscure. Many times, a luminous nebula is in fact cold, but illuminated from stars nearby. In both cases, the heat is not produced by the nebula itself, but by stars located inside or nearby. When exposed to heat and ionizing radiation (mainly ultraviolet from O - type stars and B - type stars), the gas inside a nebula becomes hot and ionized. Without a source of light and heat, the gas will cool down, its speed will decrease and it will no longer be ionized, preferring a molecular structure. Inside large nebulae, there is enough room for unseen solar systems to form and exist. In most cases, they are invisible to the world outside. Small nebulae A planetary nebula is formed from the outer shell of a dying red giant. It can reach up to 2 light years in size. At that moment, it will merge into the interstellar medium. A nebula created by a supernova will also expand up to 10 light years. From that point, it will also be slowed down and merged with the interstellar medium. Also, around young stars, neutron stars and black holes, jets of gas and dust can be seen, but they are not considered nebulae, even if they share some common properties. Small nebulae are short lived compared to the cosmic scale. Sometimes, matter from inside them might be enough to create an asteroid belt or a few planets around a stellar remnant. Stars and planets inside a nebula We know for sure that there are stars inside some nebulae. Young stars and protoplanets What is important for any future settlers is if there is anything worth terraforming or worth Industrial colonization. In case of large nebulae, many of them are the birth place for stars and planets. However, those stars are young, very hot and still accreting matter. The planets are in fact protoplanets, hot and exposed to massive bombardment as they consume more matter. There might be no or almost no suitable surface for building a base and safe from impacts. Terraforming is out of question. Even orbital stations will not be safe. Everything will be under risk of an impact. Inside a forming solar system, the only people that will want to stay are scientists, wishing to see how a solar system is created and a few adventurers, looking for adrenaline and extreme cosmic sports. Mature solar systems However, there is a possibility that a solar system can be formed earlier inside a nebula. radiation pressure from the star can keep gas at some distance. Planets should be safe, at least until enough gas accumulates at the outer border of the system to overcome radiation pressure. A planet might be safe for 1000 years and suitable for terraforming. In this scenario, terraforming is possible. But how would be life for settlers inside such a system? They will see no stars in the sky, just the nebula, illuminated by their sun. The nebula will be a major source of radio waves, affecting all communications. It will, however, have no effect on the climate of a planet. Our Sun keeps interstellar medium at over 100 AU from us. Inside a nebula, where gas pressure is far higher, solar wind and radiation pressure will not be able to keep gas so far away. Depending on what type of star it is and on how dense the nebula is, the limit will vary. Let's say it will be 7 to 10 AU. Inner rocky planets will be safe, but not the same will be said about the outer planets. They will fly inside a large radiation belt. As they move, they can accrete matter and slowly grow in size. With high amounts of ionized gas and risks of impact from coalesced clumps of matter, passing the radiation belt will be a very hard task for any spaceship. The system will be almost completely isolated from the outside world. And this is not all. From time to time, as gas pressure increases above a certain limit, matter will start to fall towards the star. Magnetic lines will force ionized gas to move towards the poles of the star, protecting the inner planets. However, we don't know what effects this will have. It is possible that this will produce solar flares or that a significant part of gas and dust will escape towards the inner planets and will create inner radiation belts. Rogue planets Small amounts of gas and dust can collide to form a rogue planet or even a rogue asteroid. Such objects will be free-floating throughout the nebula. Their gravity will force more matter to condense. Since they are small, with far less gravity then a star, they will not accrete too much matter fast and they will grow in size very slow. The process might take place slow enough that heat created in the process might not melt their core at all. The result will be a cold planet, made almost only of materials that on Earth would be in a gaseous phase: hydrogen, helium, carbon monoxide and dioxide, methane, ammonia, nitrogen, but also water and a limited amount of rocks. If the formation process produces enough heat, all materials will be separated into layers. The rocky core will be inside, followed by a large subsurface ocean, a layer of water ice and a thick layer of solidified gasses. The last on the surface will be hydrogen and helium. Rogue planets could be targeted for colonization. They would not be suitable for terraforming, since they will be made almost entirely of volatiles and there will be no source of light. Still, they might have some economical advantages. They might be the only place where helium can be mined easy. Helium 3 is proposed as an important source of energy for the future. At high cost, it can be extracted from rocks, from stars or giant planets, but here, it might be mined more easily. However, many rogue planets that never got heated wonder through the interstellar space, outside of any nebula. They should be a far better option for mining helium. Planets in small nebulae As a red giant loses matter or as a large star undergoes supernova, planets are destroyed or ejected from orbits. As it gets formed and expands outwards, the nebula will destroy planets, will blast away atmospheres and oceans (even on gas giants) and will make the solar system unrecognizable. Still, there have been identified a few planets around white dwarfs and neutron stars. They could be remnants of the former planets or they might be made from debris created by the supernova itself. Planetary and supernova nebulae are not safe places for any spacecraft to explore. If there are planets around, we have to wait at least 10000 years for the system to come to an equilibrium and then try to colonize. Problems for humans Settlers and passing ships will have major problems inside a nebula. Transportation problems Inside a nebula, matter density is small, so a spaceship can theoretically pass without major problems. The ship will be still slowed down, just like an atmospheric drag. Hot nebulae are made of ionized gas, which can destroy electronics onboard a spaceship. given the sizes of a nebula, ships will withstand long exposures. Also, there is a chance that small clumps of matter might have formed somewhere. Impacts with objects larger then 1 cm can penetrate the hull of a spaceship. Given the high risks of impact and destroying equipment, it should be recommended that spaceships should avoid passing through a nebula unless it is the only way. Another risk comes from the fact that inside a nebula you might be unable to see. Without seeing stars, any star tracker will return error data to navigation computers. Magnetic fields will not be useful, since they will fluctuate. And since radiation can destroy electronics (including other devices used for orientation, like far infrared telescopes which can find stars outside a nebula), you might find yourself flying blind. Space pirates It is questionable if space pirates and outlaws will go to a nebula to hide. First of all, they will need to go deep inside the clouds to hide themselves. Second, their ships can be destroyed by charged particles too. Since nebulae don't offer too many perspectives for settlers and industrial corporations, pirates will really have no reason to hide here. There will be almost no passing ships to hunt and almost no colonies to attack. Who will colonize a nebula? As shown above, there should be a few rare stars hosting planets suitable for terraforming. Also, those planets will be almost completely isolated from the outside world. They will be more isolated then inside space voids. So, who would like to go to such a remote place and live there? There might be a few adventurers and religious groups that will accept this task. There, unconnected to the rest of the world, they can develop their society in the way they want. Category:Galactic Structures